1. Field of the Invention
The present invention relates to an electron gun having an electrode structure with an improved focusing characteristic and to a color cathode ray tube using such an electron gun.
2. Description of the Prior Art
In general, in a color cathode ray tube such as a color cathode ray tube or a color monitor tube, to implement a so-called self-convergence function which causes a plurality of electron beams to converge on a phosphor screen without a special external correcting magnetic field being applied to them, deflection magnetic fields for deflecting the electron beams emitted from an electron gun are given predetermined distortions.
For this reason, the electron beams are subjected to deflection distortion while passing through the deflection magnetic fields, and form beam spots accompanied by haloes in the direction perpendicular to scanning lines, in the peripheral portion of the phosphor screen particularly in each corner portion of the screen. Thus, the focusing characteristic of the electron gun is degraded in such portions and, hence, the image quality is degraded.
To attain the uniformity of the focusing characteristic over the entire screen, recesses each having a rectangular shape, i.e., so-called slits, are formed in a second grid electrode of the electron gun in such a manner that electron beam apertures are respectively located in the slit-shaped recesses.
FIGS. 1(a) and 1(b) are explanatory views showing the structure of the second grid electrode used in the above-described conventional electron gun. FIG. 1(a) is a front view viewed from the first grid electrode side, and FIG. 1(b) is a cross-sectional view taken along the line I(b)--I(b) in FIG. 1(a).
An electron beam aperture 1a of a second grid electrode 2 is a circular hole which pierces the center of a slit-shaped recess 1b formed and elongated horizontally in a plate which constitutes the second grid electrode. The diameter of the electron beam aperture 1a is smaller than the length of each shorter side of the slit-like recess 1b. The slit-like recess 1b is formed by coining in the plate which constitutes the second grid electrode, so that the slit-like recess 1b has a substantially rectangular cross section which flares outwardly to a small extent from the bottom of the recess toward the open end, as shown in FIG. 1(b).
Specifically, in the shown conventional electrode structure, there is a distance d1 in the vertical direction between each longer side of the slit-like recess 1b and the edge of the electron beam aperture 1a at the bottom of the slit-like recess 1b. As a result, the effect of suppressing haloes in the vertical direction which occur during scanning of the periphery of the screen is weakened.
To solve the above-described problem, an electrode structure which will be described below has been proposed.
FIGS. 2(a) and 2(b) are explanatory views showing the structure of a second grid electrode used in another conventional electron gun. FIG. 2(a) is a front view of an electron beam aperture portion, and FIG. 2(b) is a cross-sectional view taken along the line II(b)--II(b) in FIG. 2(a).
This second grid electrode is made up of a first member 2-1 and a second member 2-2 which are fixedly bonded to each other, such as by welding. The first member 2-1 has a slit 1b formed by press cutting, while the second member 2-2 has a circular electrode beam aperture 1a having a diameter of the same size as the length of each vertical (shorter) side of the slit lb.
By adopting such an arrangement, it is possible to reduce each of the distances d1 shown in FIG. 1(a) to zero, whereby it is possible to retain the aforesaid halo suppressing effect in the peripheral portions of the screen.
To achieve a similar object, the electrode structure shown in FIGS. 3(a) and 3(b) is disclosed in Japanese Patent Laid-Open No. 60-59637 corresponding to Japanese Patent Application No. 58-164958.
FIGS. 3(a) and 3(b) are explanatory views partially showing the structure of a second grid electrode used in still another conventional electron gun. FIG. 3(a) is a front view of an electron beam aperture portion, and FIG. 3(b) is a cross-sectional view taken along the line III(b)--III(b) in FIG. 3(a).
In this electrode structure, the diameter of the electrode beam aperture 1a is greater than each shorter side of the slit-like recess 1b and the vertical wall portions of the slit-like recess 1b are shared by the electron beam aperture 1a. Accordingly, it is possible to obtain an effect similar to that described above with reference to FIGS. 2(a) and 2(b).